To a large roasting pan, add bay leaves, kosher salt, peppercorns, and wine. Add salmon and enough water to cover.
Remove salmon and set aside; bring liquid to a boil, covered. Reduce to a simmer, then gently slide salmon into liquid and simmer 10 to 15 minutes, covered, until salmon is just opaque.
While salmon is cooking, make the dressing: In a small bowl, whisk together garlic, mustard, and vinegar. Gradually whisk in oil until dressing is emulsified. Whisk in herbs and season with sea salt.
Using 2 spatulas, remove salmon, place on a cooling rack set on a sheet pan, and chill 10 minutes, loosely covered with foil. Discard poaching liquid.
Remove salmon from refrigerator and break into large pieces, discarding skin. On a very large serving platter, top watercress with pieces of fish, potatoes, eggs, green beans, olives, and tomatoes. Drizzle dressing over everything and serve immediately, accompanied by crusty bread.
Note: Nutritional analysis is per serving.